As is well known in the art of data transmission and as is described in Simon Haykin, Communication Systems, pp. 414-15 (2nd ed. 1983), clock data and the data to be transmitted may be encoded using Manchester encoding to produce a signal which includes both the data to be transmitted and the clock data. This encoding scheme is particularly useful when the transmitter and the receiver are not controlled by the same clock. In this case, the receiver may require the clock data associated with the data to be transmitted in order to recover the transmitted data. A receiver may decode a Manchester encoded signal because it includes both the data to be decoded and the associated clock data in a single transmission. Manchester encoding effectively doubles the bandwidth of the signal to be transmitted.
In addition to the clock data, a receiver needs to identify the beginning and the end of the transmission from the receiver. Systems have been developed which identify the beginning of the encoded data by transmitting an illegal Manchester code. An example of the illegal Manchester code is shown in FIG. 14. Manchester encoded data typically only includes two consecutive high data bits(e.g. "11"). The illegal code includes four consecutive high data bits (e.g. "1111"). As a result, an increased bandwidth transmitter is necessary to transmit the illegal Manchester code. Further, the illegal Manchester code includes three low data bits (e.g. "000"). As a result, the receiver may have difficulty synchronizing its internal clock. Further, the energy of the transmitted signal over a period of time is reduced. As a result, the receiver may increase its gain because of the reduced energy and, thus, reduce its signal to noise ratio.
The end of the transmission may not be identified but determined by monitoring the number of received bits. As a result, if two data packets are transmitted at the same time, it may be difficult to determine if a collision has occurred between the transmissions.